1. Technical Field
The present invention relates to a diamond electrode and a method of manufacturing the same and more particularly, to a diamond electrode for commercial use and a method of manufacturing the same by chemical vapor deposition (CVD).
2. Description of the Related Art
Known methods of forming an electrically conductive diamond layer by a chemical vapor deposition (CVD) process include methods based on hot-filament CVD, microwave plasma CVD, etc.
Pure diamond is a semiconductor having a band gap of 5.2 eV, and cannot be used as an electrode because it has little or no electrical conductivity. However, when a very small amount of boron (B) or phosphorus (P) is added during deposition of a diamond layer, an electrically conductive diamond layer is formed which can be used as an electrode. In recent years, boron-doped diamond (BDD) electrodes have been mainly used.
BDD electrodes have a large potential window and a high overvoltage for oxygen evolution as compared to other electrodes, and thus are very useful in the field of electrochemical water treatment. More specifically, BDD electrodes generate significantly large amounts of hydroxyl radicals (OH) and ozone (O3) on the electrode surface as compared to insoluble electrodes called DSA (Dimensionally Stable Anode) electrodes, and thus are very useful as electrodes for water treatment.
Furthermore, when BDD electrodes are used as electrodes for water treatment, oxidizing agents such as hydroxyl radicals (OH), ozone (O3) and hydrogen peroxide (H2O2) are produced, and potent oxidizing agents such as hypochlorite ions (OCl−) are generated in an electrolyte containing chlorine (Cl2). Accordingly, BDD electrodes can be used in the fields of electrochemical wastewater treatment, electrochemical water purification, ballast water treatment, etc.
Meanwhile, most BDD electrodes have shortcomings in that a pinhole may be present during deposition of the electrode layer, as during deposition of other layers, and the adhesion of the electrode to a substrate is reduced due to the stress of the deposited layer. Furthermore, when an amorphous carbon or graphite-like layer is formed, it has a short life span in an electrolytic environment. Moreover, the life span of the BDD electrodes may further be shortened due to the peeling of the layer either by heat generated in the electrode during the progression of an electrolytic process or the temperature of an electrolyte and chemical corrosion.
Conventionally, a method of fluorinating the surface of a substrate in order to eliminate defects, such as pinholes in a formed BDD electrode (Patent Document 1), was suggested. However, this method has disadvantages in that since the fluorination process is added, the time it takes to manufacture the BDD electrode increases to reduce productivity, and additional costs are incurred, thereby limiting the commercial use of the BDD electrode.
In addition, conventionally, there was suggested a method of forming a plurality of diamond layers having different properties, wherein a diamond layer forming the outermost surface layer of the plurality of diamond layers was formed to a thickness of 20 μm or more (see Patent Document 2). According to the disclosure of Patent Document 2, when a relatively thick diamond layer is applied, a problem may arise in that the diamond layer is peeled off due to tensile stress. To solve this problem, there was suggested a method in which a layer is also formed on the opposite surface of the substrate. However, this method has also a problem in that since the rate at which a BDD electrode layer is formed by the CVD process is only 0.1-0.7 μm/hr, the layer formation process is required to be carried out for about 100 hours in order to form the layer to a thickness of 20 μm or more.
In an attempt to overcome the above-described problems, a method was suggested in which a low-quality diamond layer was formed in the initial stage and a high-quality diamond layer was formed only in the outermost surface layer. However, this method is difficult to apply in practice because the rate at which a BDD electrode layer is deposited by a conventional CVD process is only 0.1-0.7 μm/hr and, thus, the layer deposition time is excessively long. Although the layer formation rate is somewhat proportional to the total amount of gases introduced, a method capable of forming a layer at a rate of several micrometers (μm) per hour is not yet known. In addition, the above-described method of forming a high-quality diamond layer only in the outermost surface layer still poses problems when it is commercially used.